Content moderation is a general problem, whether you're talking about child porn, content intended for mature audiences, or memes about Winnie the Pooh.
What I and others are trying to tell you is that your obsessive focus on Facebook as if they are the root cause of the problem is incorrect. There is no magic solution I'm aware of because each of them have some sort of tradeoff.
The most extreme version of content moderation I can think of is that a human being examines and approves every single message of any kind before it is published, any image of a minor is banned because it's too hard to objectively define child porn (that still leaves the open question of how to determine if someone is a minor visually), and no accounts for anyone under the age of legal majority are allowed, as verified by a legal ID that is checked by a human being.
Even in that case, kids will find some way to get an account or just use their parent's account, and the door is cracked open again. And the pedophiles will just go elsewhere, probably using a service with significantly less resources available to attack the problem, which is probably worse than the status quo.
This doesn't even touch on the privacy concerns that most people would have with every message being reviewed.
As I said before, I would welcome you to share the solution that you imply exists which addresses every issue above.